S-Adenosylhomocysteine hydrolase (E.C.3.3.1.1) from beef liver has been purified and crystallized by methods worked out by our laboratory. This enzyme appears to be homogeneous by polyacrylamide gel electrophoresis. It has a molecular weight of about 240,000 when estimated by gradient gel electrophoresis, and analytical ultracentrifugation. Six types of subunits, with molecular weights ranging between 50,000 to 60,000, were revealed by sodium dodecyl polyacrylamide gel electrophoresis. The amino acid components of this enzyme have been analyzed, and there is one glucosamine per enzyme molecule. The kinetic properties of this enzyme are currently being studied, especially with respect to chemical analogs that may substitute for the natural substrates, or act as inhibitors. The chemotherapeutic efficacies of these chemical analogs will be tested in animals and cell culture systems.